JPH10121201A - High strength spring excellent in delayed fracture resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength spring having extremely excellent delayed fracture resistance even if strength is increased to such a degree as to have >=HRC 50 material hardness after quench-and-temper treatment and also stress is increased. SOLUTION: This high strength spring has a tempered martensitic structure having >=HRC 50 hardness after quench-and-temper treatment. At this time, the amount of segregation of P in old austenitic grain boundaries, fractured in a chamber of the Auger apparatus and measured with this apparatus, is controlled to <=3.0 atomic %. As to steel components, it is desirable that this steel contains, from the standpoint of strength and toughness, 0.3-0.7% C, 0.1-3.0% Si, and 0.1-2.0% Mn and also contains 0.001-0.5% Ti and/or 0.001-0.5% Nb.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength spring used for a suspension spring for an automobile, a valve spring for an internal combustion engine, and the like. It relates to a strength spring.

[0002]

2. Description of the Related Art When manufacturing springs, JIS
(G 3565-3567 and 4801 etc.) Using a rolled material as a wire by applying hot rolling to the spring steel specified in, drawn out to a predetermined wire diameter and subjected to oil tempering Method of applying cold working to the rolled material, or subjecting the rolled material to drawing and then heating to form a spring and then quenching and tempering (hot forming)
It is common to adopt such as.

[0003] In recent years, against the background of the demand for weight reduction of various members, high stress per unit weight of springs has been required. For example, the strength after quenching and tempering is 1800 MPa.
There is a demand for high-strength steels for springs of a or higher. However, when the strength of the spring steel is increased and the hardness is increased, defects are easily generated on the surface of the spring, and moreover, the sensitivity of the defects is increased and the surroundings are easily affected. Particularly in a corrosive environment, when corrosion pits are formed on the surface due to surface defects, they become a source of stress concentration, and furthermore, the material is easily embrittled by absorbing hydrogen generated in the corrosion reaction, and as a result, the particles become grainy. It has been pointed out that delayed destruction in the world causes early cut-off.

[0004] As a method of preventing such delayed fracture, it is effective to reduce the size of crystal grains for the purpose of preventing embrittlement by hydrogen or to precipitate a fine compound. It is conceivable to add a substance forming element.

However, by adding a carbonitride forming element, huge inclusions are likely to appear, and the durability, which is a required property of the spring, tends to deteriorate. In addition, in a suspension spring used in a corrosive environment, At present, no great effect has been observed in improving the characteristics, and the increase in the stress of the spring has not progressed as expected.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a very excellent material resistance after quenching and tempering. An object of the present invention is to provide a high-strength spring exhibiting delayed fracture.

[0007]

A high-strength spring according to the present invention which has solved the above-mentioned problems has a hardness of HR after quenching and tempering.
In a high-strength spring having a tempered martensite structure of C50 or more, the point is that the amount of P segregation of the old austenite grain boundary measured in the chamber of an Auger device and broken down by the device is 3.0 at% or less. And as a steel component, from the viewpoint of strength and toughness, C:
0.3-0.7%, Si: 0.1-3.0%, Mn:
0.1 to 2.0%, and Ti: 0.001
-0.5% and / or Nb: 0.001-0.5%.

For the purpose of improving delayed fracture resistance, V
Is recommended to be added to 1.0% or less, or Ta:
One or more selected from the group consisting of 0.1% or less, Zr: 0.1% or less, and Hf: 0.1% or less may be contained.

Further, if Cr: 5.0% or less is contained as a steel component, it is effective in improving corrosion resistance. Also, Ni:
3.0% or less, Mo: 3.0% or less, and Cu: 1.0% or less.
Effective for increasing strength and improving corrosion resistance.

[0010] Further, for the purpose of improving sag resistance, Al
May be contained in an amount of 1.0% or less, or B may be contained in a range of 50 ppm or less for improving hardenability.
In addition, for the purpose of improving strength and corrosion resistance, 5.0% Co is used.
% Or less, and may contain 1.0% or less of W.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have intensively studied a spring steel having a high resistance to hydrogen generated by corrosion for the purpose of improving durability in a corrosive environment. As a result, the hardness of the material after quenching and tempering is HR
In order to improve the delayed fracture resistance of steel having a tempered martensitic structure of C50 or more, it is sufficient to disperse and precipitate fine carbonitrides by adding a carbonitride forming element such as Ti. Instead, the present inventors have found out that this can be realized only by positively preventing the segregation of P at the grain boundaries and increasing the strength of the grain boundaries, and have reached the present invention.

The major cause of the brittle fracture due to hydrogen is that the hydrogen generated by the corrosion reaction penetrates into the steel and diffuses the old austenite grain boundaries, thereby weakening the bond energy of the grain boundaries. In the present invention, by adding a carbonitride forming element such as Ti, the carbonitride is finely precipitated and acts as a hydrogen trap site to improve delayed fracture resistance. However, in the present invention, it is indispensable to increase the strength of the grain boundary in order to effectively exhibit the above-mentioned effect of trapping carbon sulfide on diffusible hydrogen. It is said that one of the causes of lowering the grain boundary strength is the segregation of P at the grain boundary. The steel containing a large amount of P has low ductility and low toughness and easily induces temper embrittlement due to the presence of P at the grain boundary. Is thought to be related to segregation.

However, the conventional method has not always revealed a clear correlation. The reason is that in the conventional method, when measuring the P content of the grain boundary, it is general to analyze by X-ray diffraction, and it is difficult to measure only P segregated at the grain boundary, This is considered to be due to the measurement of P in the crystal grains.

Therefore, the inventors of the present invention determine that the amount of P in the former austenite grain boundary is obtained after the steel which has been quenched and tempered in the chamber of the Auger apparatus undergoes grain boundary destruction in which hydrogen is absorbed and destroyed. The effect of P content in steel on delayed fracture and the effect of P segregation of prior austenite grain boundaries (hereinafter simply referred to as grain boundaries) on delayed fracture were investigated by a method. As a result, it has been found that the delayed fracture susceptibility is poorly correlated with the total P content in the steel, and that the segregation amount of P at the grain boundaries has a substantial adverse effect on the delayed fracture susceptibility.
FIG. 1 shows spring steel Nos. Used in Examples described later.
The results of examining delayed fracture life for 1, 2, and 3 by changing the amount of segregation of grain boundary P are shown.

It is understood that the delayed fracture resistance can be enhanced by setting the segregation amount of the grain boundary P to 3.0 at% or less. The segregation amount of grain boundary P is desirably 2.0 at% or less,
It is more preferable that the content is 1.0 at% or less. In particular, in steels having a high C content and low material toughness, the tendency becomes remarkable when the grain boundary segregation amount of P is 1.0 at% or more.
Note that the fracture modes were all grain boundary fractures at the former austenite grain boundary.

The present invention does not limit the method for controlling the amount of segregation of P at the grain boundary, but can be achieved by combining the following methods. The amount of P segregated at the grain boundary depends on the size of the crystal grains as well as the component composition.

Although the amount of P in steel does not always correlate with delayed fracture resistance, it is effective to reduce the amount of P as a whole in reducing the amount of segregation of P at the grain boundary. P
It is recommended that the amount be reduced. Specifically, the amount of P in the steel at the steelmaking stage is preferably set to 0.030% by weight or less, and more preferably controlled to 0.015% by weight or less.

Further, as the crystal grain size becomes smaller, delayed fracture is less likely to occur even if the amount of P grain boundary segregation becomes larger. On the other hand, when the crystal grain size is larger, delayed fracture occurs even when the amount of P grain boundary segregation is smaller. Is easy to occur. Therefore, the prior austenite crystal grain size is desirably 50 μm or less,
It is more desirable that the thickness be less than μm. Also, if the quenching temperature is too high, the prior austenite crystal grain size increases,
In addition, the segregation amount of P at the grain boundary is increased, and delayed fracture is likely to occur. Therefore, the quenching temperature is desirably in the austenitizing temperature range of 1000 ° C. or less.

The grain boundary segregation of P can be prevented by setting the austenitizing temperature at the time of hot rolling or spring production to 1000 ° C. or less. This also prevents the austenite crystal grain size from becoming coarse, thereby preventing a decrease in grain boundary strength. Next, the reasons for limiting the chemical components in the high-strength spring steel according to the present invention will be described.

C: 0.3 to 0.7% C is an element indispensable for securing strength (hardness) after quenching and tempering, and is required to be 0.3% or more, and 0.35% It is preferable that it is above. However, if added in a large amount, not only deteriorates the toughness and ductility after quenching and tempering, but also adversely affects the corrosion resistance, so the upper limit was made 0.7%. It is preferably at most 0.55%, more preferably at most 0.5%.

Si: 0.1 to 3.0% Si is a solid solution strengthening element. If the content is too small, the strength of the matrix becomes insufficient.
It is preferable that it is 1.0% or more. However, if added in excess, the carbides will not sufficiently dissolve during quenching and heating, so heating in a high temperature range is required to uniformly austenitize, resulting in excessive decarburization on the surface. The fatigue characteristics of the spring deteriorate. Therefore, the Si addition amount needs to be 3.0% or less, desirably 2.5% or less, and more preferably 2.0% or less.

Mn: 0.05 to 2.0% Since Mn is an element for improving hardenability, 0.05 %
% Must be added. If the amount is too large, a large amount of retained austenite appears during quenching and it becomes impossible to obtain a predetermined strength and hardness. Therefore, the upper limit is set to 2.0%. Furthermore, since Mn tends to promote delayed fracture,
In the case of containing many elements such as Si and Si (in the case of high strength and high hardness), the content is preferably 1.0% or less, and 0.5% or less.
The following is more desirable.

Ti: 0.001 to 0.5% Nb: 0.001 to 0.5% Ti and Nb are carbonitride forming elements that precipitate a large amount of ultrafine carbonitrides in grains and at grain boundaries. When P segregation at the grain boundary is small, by acting as a trap site for diffusible hydrogen harmful to delayed fracture, the delayed fracture resistance can be remarkably improved. Further, the carbonitride can reduce the size of crystal grains, improve the toughness of the material, and improve the sag resistance of the spring. Such an effect is observed when the content is 0.001% or more. However, in order to exhibit a more remarkable effect, 0.005% or more is required.
% Or more is desirable.

However, if the amount of addition is too large, coarse carbonitrides are formed during the solidification process, and if a large amount of coarse carbonitrides is formed, the fatigue characteristics of the spring are significantly deteriorated. Therefore, the content needs to be 0.5% or less, preferably 0.3% or less, and more preferably 0.1% or less.

V: 1.0% or less (excluding 0%) Ta: 0.1% or less (excluding 0%) Zr: 0.1% or less (excluding 0%) Hf: 0.1 % Or less (excluding 0%) V, Ta, Zr, and Hf all exert the same effect on delayed fracture resistance, and 0.001%
It is desirable to add above, and more desirable to add 0.005% or more. However, the upper limit should be set according to the type of element. In the case of V, the effect is exhibited by adding a relatively large amount.
When the content is in the range of 1.0% or less, the above-described effect can be obtained without generating coarse and large amounts of inclusions harmful to fatigue breakage. Further, with respect to Ta, Zr, and Hf, if the amount is too large, coarse carbonitrides are generated, thereby impairing the fatigue characteristics. Therefore, it is desirable to set the upper limit to 0.1% or less. Further, in the present invention, Cr, Ni, Mo, C
By adding elements such as u, Al, B, Co, and W, it is possible to improve characteristics other than fatigue fracture resistance.

Cr: 5.0% or less (excluding 0%) Cr is an element having an effect of improving corrosion resistance and contributing to an improvement in hardenability similarly to Mn. In order to effectively exert such an effect, it is desirable to add 0.05% or more. However, if the amount is too large, the carbides are less likely to dissolve during quenching and heating, and the strength and hardness are rather reduced. Therefore, the content needs to be 5.0% or less. Since Cr also has the effect of lowering the material toughness, it is recommended that the upper limit be 1.5%. Ni,
Mo and Cu may be added for the purpose of increasing strength and improving corrosion resistance.

Ni: 3.0% or less (excluding 0%) Ni has an effect of improving the toughness and corrosion resistance of the material after quenching and tempering, and also has an effect of significantly improving the sag property, which is important as a spring property. Have. In order to effectively exert these effects, it is preferable to add 0.05% or more. However, even if the addition amount is too large, the hardenability increases and a supercooled structure easily appears after rolling, so the upper limit is preferably 3.0%, more preferably 1.0% or less.

Mo: 3.0% or less (excluding 0%) Mo is an element that improves hardenability and forms molybdate ions during corrosion dissolution to enhance corrosion resistance. Further, the effect of increasing the grain boundary strength to improve delayed fracture resistance is also recognized. In order to improve hardenability and delayed fracture resistance, it is necessary to add at least 0.05% or more. 3.
Addition of 0% or more not only saturates the effect,
Since it is an expensive element, the content is set to 3.0% or less.

Cu: 1.0% or less (excluding 0%) Cu is an element electrochemically more noble than iron, and has an effect of densifying generated rust and increasing corrosion resistance. Such an effect is effectively exhibited by adding 0.01% or more.
Even if the content exceeds 0%, no further effect is obtained,
Rather, the material may be embrittled during hot rolling,
It is recommended to add in the range of 1.0% or less. Further, in the spring steel of the present invention, Al, B, Co, W may be added within the following range.

Al: 1.0% or less (excluding 0%) Al is an element that refines crystal grains to improve a proof stress ratio and contributes to an improvement in sag resistance. Such an effect is 0.
Effectively exhibited by addition of 005% or more. However,
Even if added in excess of 1.0%, no further effect is obtained, but rather a large amount of oxide-based inclusions (such as Al ₂ O ₃ ) is formed and coarsened, and the delayed fracture life is rather short. Therefore, the upper limit is desirably set to 1.0%.

B: 50 ppm or less (excluding 0 ppm )
B ) Since B is an element that improves hardenability by adding a small amount and increases grain boundary strength, it is recommended to contain B in an amount of 1 ppm or more. The effect of improving hardenability saturates even if 50 ppm or more is added, so the addition amount may be 50 ppm or less.

Co: 5.0% or less (excluding 0%) Co is an element that improves the strength and hardness after quenching and tempering, and also densifies generated rust to improve corrosion resistance. % Or more is desirable. However, if the amount is too large, the effect is saturated and it is also an expensive element. Therefore, the addition amount is preferably set to 5.0% or less.

W: 1.0% or less (excluding 0%) W improves strength and hardness after quenching and tempering,
Since it is an element which forms tungstate ions during corrosion dissolution to increase corrosion resistance, it is preferable to add 0.01% or more. However, if added excessively, the toughness of the material tends to decrease, so that it is preferably 1.0% or less.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and are carried out with appropriate modifications within a range that can conform to the above and subsequent points. All of these are included in the technical scope of the present invention.

[0035]

EXAMPLES Steels having various chemical components shown in Table 1 were melted in a vacuum melting furnace.

[0036]

[Table 1]

The obtained steel is hot forged, quenched and tempered so that the HRC becomes 50 or more, and a plate-shaped four-point bending test piece (65 mm × 15 mm × 1.5 mm) is prepared by machining. did. The time until the test piece was broken was measured by performing a four-point bending test on the obtained test piece in a cathode charge environment.

On the other hand, the amount of P segregation at the grain boundary was measured by cutting out from a four-point bending test piece and preliminarily absorbing a large amount of hydrogen by cathodic charging to break the grain boundary in the chamber of the Auger apparatus. The amount of P segregated at the grain boundary was measured by a method of measuring the amount of P segregated in P. Tables 2 and 3 show the results of hardness, P content in steel, P segregation at grain boundaries, and delayed fracture life after quenching and tempering.

[0039]

[Table 2]

[0040]

[Table 3]

No. 1a to 1d, No. 2a-2d, N
o. 3a to 3d are the same components except for the P content. The results in Table 2 show that the delayed fracture life has a higher correlation with the amount of P segregation at the grain boundary than the P content in steel.

No. 3 in Table 3 Nos. 4 to 14 are examples of the present invention, and have a hardness after quenching and tempering of 50 or more in terms of HRC and are excellent in delayed fracture resistance. No. 15,16
Is a conventional example (JIS SUP7, SAE9254), and it can be seen that the delayed fracture life is short. No. 17, 19 and 21 are comparative examples not containing Ti or Nb, and have a short delayed fracture life. No. Nos. 18, 20, and 22 are comparative examples in which the amounts of C, Si, and Mn are too small, and a predetermined hardness cannot be obtained. No. Nos. 23 to 26 are comparative examples in which the amounts of Mn, Ni, Cr, and Mo were too high, and HRC was less than 50 due to generation of a large amount of retained austenite, and the predetermined strength was not obtained.

[0043]

As described above, according to the present invention, there is provided a high-strength spring which exhibits extremely excellent delayed fracture resistance even if the material hardness after quenching and tempering is increased to HRC 50 or more. It can be done.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of segregation of grain boundaries P and delayed fracture life of spring steel.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takashi Iwata 1-5-5 Takatsukadai, Nishi-ku, Kobe City Inside Kobe Research Institute, Kobe Steel Ltd. (72) Inventor Hiroshi Ieguchi 1-chome, Takatsukadai, Nishi-ku, Kobe-shi No. 5-5 Kobe Steel, Ltd.Kobe Research Institute (72) Inventor Shigenobu Namba 1-5-5 Takatsukadai, Nishi-ku, Kobe City, Kobe Steel Research Institute Kobe Research Institute

Claims (6)

[Claims] The hardness after quenching and tempering is HRC50.
In the high-strength spring having a tempered martensitic structure as described above, the amount of P segregation of the prior austenite grain boundaries measured in the chamber of the Auger device after being broken is 3.0 at%.
A high-strength spring having excellent delayed fracture resistance, characterized in that: 2. C: contains 0.3 to 0.7% (mass%, the same applies hereinafter), Si: 0.1 to 3.0%, Mn: 0.1 to 2.0%, and Ti : 0.001 to 0.5% and / or Nb: 0.00
2. The high-strength spring according to claim 1, comprising a steel containing 1 to 0.5%. 3. The steel according to claim 1, further comprising V: 1.0% or less (0%).
% Of the high-strength spring according to claim 2. 4. The steel according to claim 1, further comprising Ta: 0.1% or less (0%).
%, Zr: 0.1% or less (excluding 0%), Hf: 0.1% or less (excluding 0%), at least one selected from the group consisting of: 4. The high-strength spring according to 2 or 3. 5. The steel according to claim 5, further comprising Cr: 5.0% or less (0%).
% Of the high-strength spring according to any one of claims 2 to 4. 6. Ni: 3.0% or less (excluding 0%), Mo: 3.0% or less (excluding 0%), Cu: 1.0% or less (excluding 0%) The high-strength spring according to any one of claims 2 to 5, comprising at least one member selected from the group consisting of: